A computer network is a collection of interconnected computing devices that can exchange data and share resources. Example network devices include layer two devices that operate within the second layer (L2) of the Open Systems Interconnection (OSI) reference model, i.e., the data link layer, and layer three devices that operate within the third layer (L3) of the OSI reference model, i.e., the network layer. Network devices within computer networks often include a control unit that provides control plane functionality for the network device and forwarding components for routing or switching data units.
An Ethernet Virtual Private Network (EVPN) may be used to extend two or more remote layer two (L2) customer networks through an intermediate layer three (L3) network (usually referred to as a provider network), in a transparent manner, i.e., as if the intermediate L3 network does not exist. In particular, the EVPN transports L2 communications, such as Ethernet packets or “frames,” between customer networks via traffic engineered label switched paths (LSP) through the intermediate network in accordance with one or more multiprotocol label switching (MPLS) protocols. In a typical configuration, provider edge (PE) devices (e.g., routers and/or switches) coupled to the customer edge (CE) network devices of the customer networks define label switched paths (LSPs) within the provider network to carry encapsulated L2 communications as if these customer networks were directly attached to the same local area network (LAN). In some configurations, the PE devices may also be connected by an IP infrastructure in which case IP/GRE tunneling or other IP tunneling can be used between the network devices.
In an EVPN, MAC learning between PE devices occurs in the control plane rather than in the data plane (as happens with traditional bridging) using a routing protocol. For example, in EVPNs, a PE device typically uses the Border Gateway Protocol (BGP) (i.e., an L3 routing protocol) to advertise to other provider edge network devices the MAC addresses learned from the local customer edge network devices to which the PE device is connected. A PE device may use BGP route advertisement message to announce reachability information for the EVPN, where the BGP route advertisement specifies one or more MAC addresses learned by the PE device instead of L3 routing information.
In an EVPN configuration referred to as the active-standby EVPN multi-homing mode of operation, an Ethernet segment includes multiple PE devices that provide multi-homed connectivity for one or more local customer edge (CE) devices. Moreover, the multiple PE device provide transport services through the intermediate layer 3 network to a remote PE device, and each of the multiple PE devices in the Ethernet segment forwards Ethernet frames in the segment for the CE device. In the active-standby EVPN multi-homing mode of operation, only a single active PE router of the multi-homing PE routers is allowed to forward traffic to and from an Ethernet segment that make up the set of L2 links connecting the multi-homed CE device with the multi-homing PE routers. When a network failure occurs at the active one of the multiple PE devices that provide multi-homed connectivity, the active PE device assumes a role of a standby PE device and one of the standby PE devices switches to become the active PE device.